1,081 research outputs found
Far and extreme ultraviolet radiation fields and consequent disc destruction in star-forming regions
The first stages of planet formation usually occur when the host star is still in a (relatively) dense star-forming region, where the effects of the external environment may be important for understanding the outcome of the planet formation process. In particular, star-forming regions that contain massive stars have strong far-ultraviolet (FUV) and extreme ultraviolet (EUV) radiation fields, which can induce mass-loss from protoplanetary discs due to photoevaporation. In this paper, we present a parameter-space study of the expected FUV and EUV fields in N-body simulations of star-forming regions with a range of initial conditions. We then use recently published models to determine the mass-loss due to photoevaporation from protoplanetary discs. In particular, we focus on the effects of changing the initial degree of spatial structure and initial virial ratio in the star-forming regions, as well as the initial stellar density. We find that the FUV fields in star-forming regions are much higher than in the interstellar medium, even when the regions have stellar densities as low as in the Galactic field, due to the presence of intermediate-mass, and massive, stars (>5 M⊙). These strong radiation fields lead to the destruction of the gas component in protoplanetary discs within 1 Myr, implying that gas giant planets must either form extremely rapidly (<1 Myr), or that they exclusively form in star-forming regions like Taurus, which contain no intermediate-mass or massive stars. The latter scenario is in direct tension with meteoritic evidence from the Solar system that suggests the Sun and its protoplanetary disc was born in close proximity to massive stars
Gyroid cuticular structures in butterfly wing scales: biological photonic crystals
We present a systematic study of the cuticular structure in the butterfly wing scales of some papilionids (Parides sesostris and Teinopalpus imperialis) and lycaenids (Callophrys rubi, Cyanophrys remus, Mitoura gryneus and Callophrys dumetorum). Using published scanning and transmission electron microscopy (TEM) images, analytical modelling and computer-generated TEM micrographs, we find that the three-dimensional cuticular structures can be modelled by gyroid structures with various filling fractions and lattice parameters. We give a brief discussion of the formation of cubic gyroid membranes from the smooth endoplasmic reticulum in the scale's cell, which dry and harden to leave the cuticular structure behind when the cell dies. The scales of C. rubi are a potentially attractive biotemplate for producing three-dimensional optical photonic crystals since for these scales the cuticle-filling fraction is nearly optimal for obtaining the largest photonic band gap in a gyroid structure
Rapid destruction of protoplanetary discs due to external photoevaporation in star-forming regions
We analyse N-body simulations of star-forming regions to investigate the effects of external far- and extreme-ultraviolet photoevaporation from massive stars on protoplanetary discs. By varying the initial conditions of simulated star-forming regions, such as the spatial distribution, net bulk motion (virial ratio), and density, we investigate which parameters most affect the rate at which discs are dispersed due to external photoevaporation. We find that disc dispersal due to external photoevaporation is faster in highly substructured star-forming regions than in smooth and centrally concentrated regions. Subvirial star-forming regions undergoing collapse also show higher rates of disc dispersal than regions that are in virial equilibrium or are expanding. In moderately dense (∼100 M⊙ pc−3) regions, half of all protoplanetary discs with radii ≥100 au are photoevaporated within 1 Myr, three times faster than is currently suggested by observational studies. Discs in lower density star-forming regions (∼10 M⊙ pc−3) survive for longer, but half are still dispersed on short time-scales (∼2 Myr). This demonstrates that the initial conditions of the star-forming regions will greatly impact the evolution and lifetime of protoplanetary discs. These results also imply that either gas giant planet formation is extremely rapid and occurs before the gas component of discs is evaporated, or gas giants only form in low-density star-forming regions where no massive stars are present to photoevaporate gas from protoplanetary discs
The Magnetic Sun: Reversals and Long-Term Variations
A didactic introduction to current thinking on some aspects of the solar
dynamo is given for geophysicists and planetary scientists.Comment: 17 pages, 9 figures; Space Science Rev., in pres
Evolution of active and polar photospheric magnetic fields during the rise of Cycle 24 compared to previous cycles
The evolution of the photospheric magnetic field during the declining phase
and minimum of Cycle 23 and the recent rise of Cycle 24 are compared with the
behavior during previous cycles. We used longitudinal full-disk magnetograms
from the NSO's three magnetographs at Kitt Peak, the Synoptic Optical Long-term
Investigations of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM), the
Spectromagnetograph and the 512-Channel Magnetograph instruments, and
longitudinal full-disk magnetograms from the Mt. Wilson 150-foot tower. We
analyzed 37 years of observations from these two observatories that have been
observing daily, weather permitting, since 1974, offering an opportunity to
study the evolving relationship between the active region and polar fields in
some detail over several solar cycles. It is found that the annual averages of
a proxy for the active region poloidal magnetic field strength, the magnetic
field strength of the high-latitude poleward streams, and the time derivative
of the polar field strength are all well correlated in each hemisphere. These
results are based on statistically significant cyclical patterns in the active
region fields and are consistent with the Babcock-Leighton phenomenological
model for the solar activity cycle. There was more hemispheric asymmetry in the
activity level, as measured by total and maximum active region flux, during
late Cycle 23 (after around 2004), when the southern hemisphere was more
active, and Cycle 24 up to the present, when the northern hemisphere has been
more active, than at any other time since 1974. The active region net proxy
poloidal fields effectively disappeared in both hemispheres around 2004, and
the polar fields did not become significantly stronger after this time. We see
evidence that the process of Cycle 24 field reversal has begun at both poles.Comment: Accepted for publication in Solar Physic
Probing the Environment with Galaxy Dynamics
I present various projects to study the halo dynamics of elliptical galaxies.
This allows one to study the outer mass and orbital distributions of
ellipticals in different environments, and the inner distributions of groups
and clusters themselves.Comment: 5 pages, 2 figs, to appear in Proc. ESO Workshop, Groups of Galaxies
in the Nearby Universe (5-9 Dec 2005), eds. I. Saviane, V. Ivanov & J.
Borissova (Springer-Verlag
Off-Diagonal Elements of the DeWitt Expansion from the Quantum Mechanical Path Integral
The DeWitt expansion of the matrix element M_{xy} = \left\langle x \right|
\exp -[\case{1}{2} (p-A)^2 + V]t \left| y \right\rangle, in
powers of can be made in a number of ways. For (the case of interest
when doing one-loop calculations) numerous approaches have been employed to
determine this expansion to very high order; when (relevant for
doing calculations beyond one-loop) there appear to be but two examples of
performing the DeWitt expansion. In this paper we compute the off-diagonal
elements of the DeWitt expansion coefficients using the Fock-Schwinger gauge.
Our technique is based on representing by a quantum mechanical path
integral. We also generalize our method to the case of curved space, allowing
us to determine the DeWitt expansion of \tilde M_{xy} = \langle x| \exp
\case{1}{2} [\case{1}{\sqrt {g}} (\partial_\mu - i
A_\mu)g^{\mu\nu}{\sqrt{g}}(\partial_\nu - i A_\nu) ] t| y \rangle by use of
normal coordinates. By comparison with results for the DeWitt expansion of this
matrix element obtained by the iterative solution of the diffusion equation,
the relative merit of different approaches to the representation of as a quantum mechanical path integral can be assessed. Furthermore, the
exact dependence of on some geometric scalars can be
determined. In two appendices, we discuss boundary effects in the
one-dimensional quantum mechanical path integral, and the curved space
generalization of the Fock-Schwinger gauge.Comment: 16pp, REVTeX. One additional appendix concerning end-point effects
for finite proper-time intervals; inclusion of these effects seem to make our
results consistent with those from explicit heat-kernel method
Acoustic Power Absorption and its Relation with Vector Magnetic Field of a Sunspot
The distribution of acoustic power over sunspots shows an enhanced absorption
near the umbra--penumbra boundary. Earlier studies revealed that the region of
enhanced absorption coincides with the region of strongest transverse potential
field. The aim of this paper is to (i) utilize the high-resolution vector
magnetograms derived using Hinode SOT/SP observations and study the
relationship between the vector magnetic field and power absorption and (ii)
study the variation of power absorption in sunspot penumbrae due to the
presence of spine-like radial structures. It is found that (i) both potential
and observed transverse fields peak at a similar radial distance from the
center of the sunspot, and (ii) the magnitude of the transverse field, derived
from Hinode observations, is much larger than the potential transverse field
derived from SOHO/MDI longitudinal field observations. In the penumbra, the
radial structures called spines (intra-spines) have stronger (weaker) field
strength and are more vertical (horizontal). The absorption of acoustic power
in the spine and intra-spine shows different behaviour with the absorption
being larger in the spine as compared to the intra-spine.Comment: 18 pages, 7 figures, In Press Solar Physics, Topical Issue on
Helio-and-Astroseismolog
Indirect study of low-energy resonances in P31(p,)28Si and Cl35(p,)32S
The reaction sequences governing the reaction flow in the rp process are important for the understanding of the energy generation and nucleosynthesis of heavy elements in hot and explosive stellar hydrogen burning. Of considerable interest are (p,) reactions along the process path which lead to the formation of reaction cycles rather than to chains of proton capture processes and decays. Previous direct attempts to measure the low-energy reaction cross sections for P31(p,)28Si and Cl35(p,)32S resulted only in upper limits for the strengths of possible low-energy resonances which may dominate the reaction rates. In this paper an indirect experimental approach is presented to study the structure of the low-energy unbound states in the compound nuclei 32 S and Ar36. The results allow a more accurate determination of the contributions of these low-energy levels in the (p,) reaction channel
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